The evolution of brake systems for heavy duty applications, such as off-the-road vehicles, has largely seen the piecemeal assemblage of oversized components from systems designed for smaller vehicles. Attempts have been made to employ air, hydraulic or combined systems in such a fashion as to provide braking characteristics required for a specific vehicle with auxiliary and back up elements as required for a particular application. None of these systems employed to data have achieved broad commercial acceptance in regard to a variety of types of off-the-road vehicles.
Perhaps the most common type of such braking systems is essentially an oversized conventional air brake system provided with multiple control valves and multiple reservoirs to produce suitable braking capacity for the vehicle involved. These oversized air systems have the disadvantages of requiring large volumes of low pressure air, providing reaction time delays, and incorporating numerous relay valves and auxiliary tanks. Such air brake systems present substantial compressor requirements with attendant disadvantages such as cold weather problems in the nature of moisture freezing in the air lines. Auxiliary dryers and filters for reducing moisture and eliminating contamination are necessary, together with regular maintenance to prevent air system degradation or failure. In addition, air systems inherently provide nonlinear braking response and do not in the absence of uncommon sophistication provide pedal "feel."
In the case of combined air and hydraulic systems such as air hydraulic intensifier systems all of the normal air circuitry is required as well as a variety of hydraulic components. Thus, dual maintenance capabilities with respect to both air and hydraulic systems are required to maintain these systems. In addition, if master cylinders are used in such systems low fluid or brake misadjustment can result in insufficient fluid displacement and loss of braking capability even though the air control circuitry may be functioning satisfactorily. To provide a complete warning capability in these systems requires duplication of sensors in the hydraulic circuit to signal system degradation or failure.
Existing hydraulic systems for these applications have generally followed the approach of oversizing systems designed for smaller, lower load applications. In this respect, oversizing can result in extremely high brake pedal pressures, oversized system components and lack of system flexibility in handling secondary warning and back up features. In addition, poppet valves and other element arrangements which may be satisfactory for small, relatively low pressure systems may in some instances be ultra sensitive or lacking in flexibility in their adaptation to large system requirements.